A photovoltaic (PV) cell is a semiconductor transducer for converting visible (about 400-700 nm) or near infrared (about 750-1000 nm) wavelengths of the electromagnetic spectrum into electrical energy. Photovoltaic cells are used in terrestrial and space power arrays to generate electric power. Lightweight space power arrays are in increasing demand as commercial, global telecommunication systems involving a constellation of satellites in low, medium, or geosynchronous orbits are necessary for relaying the RF communication signals. Reducing weight and cost for these relay satellites is important to the overall commercial success of these ventures.
Concentrator photovoltaic cells or arrays use focusing optics to concentrate or intensify incident solar radiation from a strength of one sun to many suns, i.e., on the order of 50-1,000 or more suns. Because of the concentration, the required active area of the cells is reduced. Concentrator cells are especially useful for space applications where the absolute mass of the array and its specific power (i.e., power per unit mass) is of greater concern. Typical Boeing concentrator solar cells and modules are described in U.S. Pat. Nos. 5,217,539; 5,123,968; 5,118,361; and 5,096,505, which I incorporate by reference.
Concentrator photovoltaic cells require precise alignment of the optical assembly with the sun (i.e., the incident radiation). Otherwise, the conversion efficiency suffers because the concentrating lens or lenses cause the light to miss the PV cell. For example, for a concentrator cell using Entech's silicone Fresnel concentrating lens (see U.S. Pat. No. 5,096,505), the cell generates power efficiently only within a range of .+-.2.degree. off absolute alignment with the incident radiation. If misalignment exceeds 4.degree., power generation drops essentially to zero. Between 2.degree. and 4.degree. of offset, the conversion efficiency declines markedly. The array is designed to have the minimal mass to supply the necessary power. So a decline in conversion efficiency translates to loss of mission capability. Therefore, with these systems, accurate tracking is crucial. Controlling the alignment requires a sophisticated tracking system to realign the cell or array as the relationship of the sun and satellite changes because of the earth's rotation and the orbiting of the satellite. Allowing a wider tracking angle through use of a concentrating coverglass will provide a significant advantage over conventional concentrator arrays.
To relax the tracking angle, one prior art system used a large secondary lens between a primary lens (like that of U.S. Pat. No. 5,096,505) and the cell. This secondary lens included a domed upper surface and inclined sidewalls. The secondary lens relied upon internal reflection at its sidewalls to redirect radiation toward the active area of the cell. These secondary lenses impose, in space applications, a mass penalty that compromises the benefit gained from their use.
Entech developed a prismatic cover slide to reduce or to eliminate wire grid reflection losses. The prismatic cover slide redirects incident light rays away from the wire grid lines and into the cell active area, thereby increasing the conversion efficiency. Without the prismatic cover slide, the incident radiation that hit the grid lines would be absorbed or reflected instead of being connected to electrical energy in the PV cell. Further details concerning this prismatic cover slide are provided in U.S. Pat. No. 5,217,539.